Explicitly disallow USB mass storage devices in WebUSB

In crrev.com/c/2548705, we disabled access to USB mass storage devices
to all clients aside from VMs in the USB service. However, as these were
formerly accessible from chrome.usb when allowed in policy, we want to
push this logic up into the consumers of the USB service.

While Blink already disallows claiming mass storage interfaces, this CL
ensures we keep the intended behaviour by having the browser process
hide mass storage devices from the renderer process entirely.

Bug: 1169558
Change-Id: I36a41fda581870bb4131234ae82ca5bedea360d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2670379


Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Timothy Loh <timloh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#850442}